An image recording apparatus in which small droplets of liquid ink are ejected onto a recording medium to form a dot image has been put to practical use as an ink jet printer. The ink jet printer is attracting attention as a recording means on plain paper for its advantages over other recording means, such as low noise, compactness, and no necessity of development and fixing. Of various ink jet printing systems so far invented, typical are a system comprising ejecting ink droplets under pressure of vapors generated by heat of a heating element (see, for example, JP-B-56-9429 and JP-B-61-59911 (The term "JP-B" used herein means an "examined Japanese patent publication")) and a system comprising ejecting ink droplets by mechanical pulsated pressure generated by a piezoelectric element.
Recording heads practically used in these ink jet printers include a serial scanning type head which is mounted on a carriage and moves in a direction (main scanning direction) perpendicular to the moving direction of recording paper (secondary scanning direction). Because it is difficult to increase the recording speed with this serial scanning type head, a so-called line scanning type head is conceivable, which is as wide as recording paper to achieve an increased recording speed. However, it is not easy to realize such a line scanning type head for the following reasons.
An ink jet recording system essentially suffers from local concentration of ink due to evaporation or volatilization of the solvent, which is apt to cause clogging of individual fine nozzles in conformity to resolving power. Further, the system utilizing pressure of vapors to create an ink jet involves adhesion of insoluble matter due to thermal or chemical reactions between the vapors and the ink, and the system using pressure generated by a piezoelectric element has a complicated structure for ink passage that may boost the likelihood of clogging. A line scanning type head requires several thousands of nozzles, much more than several tens to a hundred and several tens of nozzles as required in a serial scanning type head. Therefore, clogging occurs with fairly high frequency in the line scanning type head to reduce reliability seriously.
Another problem associated with conventional ink jet recording apparatus is inadequacy for improvement of resolving power. That is, the system using the pressure of vapors has difficulty in forming ink droplets of 20 .mu.m or smaller in diameter. Since ink droplets unavoidably feather on a recording medium, it is difficult for ink droplets greater than 20 .mu.m to form dots of about 50 .mu.m or smaller in diameter. On the other hand, because the recording head used in the system utilizing the pressure generated from a piezoelectric element has a complicated structure, there is difficulty of processing in making a head of high resolving power.
In order to overcome these disadvantages, an ink jet recording method involving use of electrostatic force has been proposed, in which a voltage is applied to an array of thin film electrodes to eject ink or a colorant component thereof from the ink liquid surface. Included in this method are a system comprising ejecting ink by electrostatic attraction force (see JP-A-49-62024 and JP-A-56-4467 (The term "JP-A" used herein means an "unexamined published Japanese patent application")) and a system comprising ejecting ink containing charged colorant particles in an increased concentration (see WO93/11866: PCT/AU92/00665).
In these systems a slit nozzle head that does not require a nozzle for every dot or a nozzleless head that does not require a partition for making up an ink channel for every dot is used. Therefore, the systems are effective in preventing or treating clogging that has been a great bar to realization of a line scanning type recording head. According to the latter system, in particular, ink droplets of very small diameter can be formed and ejected stably, and it is colorant particles in ink that is mainly ejected so that the printed material suffers little feathering. The latter system is therefore promising for achieving high resolving power. Conventional ink jet recording apparatus using the system of ejecting colorant particles by electrostatic force comprise, as stated above, a recording head having an array of electrodes, on each of which a high voltage of several hundreds to several thousands of volts is applied to eject ink.
However, in case where such an electrode array is applied to a multi-nozzle head having a plurality of ink ejection nozzles like a line scanning type head, electric field interference between adjacent electrodes gives rise to a problem when a high voltage is fed to the electrodes. Further, the voltage signals for ink ejection are generally sent by on-off control of the output voltage from a high-voltage power source by means of a drive. Where the applied voltage is high, it is necessary to use a high-voltage resistant drive, which increases the price of the apparatus. Although this problem could be solved by reducing the distance between the head and a recording medium to lower the applied voltage, such will make image formation instable, because the printing performance is liable to undergo the influences of the thickness of recording medium, paper dust, or dust in air.